python 中 "if x is not None" 和 "if not x is None" 有什么区别？

Question

google 的 python 风格指南中这么说：

Beware of writing if x: when you really mean if x is not None:—e.g., when testing whether a variable or argument that defaults to None was set to some other value. The other value might be a value that's false in a boolean context!

也就是说，推荐使用 if x is not None 进行判断，but why?

Answer 1

The content does not match the title. if x and if not x is None are different.

if x will make a __nonzero__ judgment on x. When x is '' (empty string), {} (empty dictionary), or 0, it will be False. When you really want to determine that a variable is not None, you should use if x is not None.

As for if not x is None and if x is not None are the same, just choose the one that sounds smooth to you.

Answer 2

The former is closer to vernacular, while the latter may cause readers to misunderstand it as if (not x) is None.

Answer 3

First, check the opcode directly (XP+Python3.4).
Opcode for x is not None:

dis.dis('if x is not None: pass')
     0 LOAD_NAME               0 (x)
     3 LOAD_CONST              0 (None)
     6 COMPARE_OP              9 (is not)
     9 POP_JUMP_IF_FALSE       15
    12 JUMP_FORWARD            0 (to 15)
>>  15 LOAD_CONST              0 (None)
    18 RETURN_VALUE

if not x is None opcode:

dis.dis('if not x is None: pass')
     0 LOAD_NAME                0 (x)
     3 LOAD_CONST               0 (None)
     6 COMPARE_OP               9 (is not)
     9 POP_JUMP_IF_FALSE       15
    12 JUMP_FORWARD             0 (to 15)
>>  15 LOAD_CONST               0 (None)
    18 RETURN_VALUE

As you can see, the operation codes are the same!
The questioner can also test the situation of adding or or and at the end.

Personally, if x is not None is easier to read than if not x is None. After all, there is a isn't in English.

Answer 4

if not x is None and if x is not None are the same to the computer. It's different for humans.

The implicit meaning of the former is that x should be None but is not, and the latter is that x should not be None but is not. Personal feeling, no objective basis (it seems no one has done such a psychological experiment?).

Answer 5

The former is more pythonic

Answer 6

If your original x is None
You need to execute the following code to determine whether x has changed or is it still None?

x = False
if x:
    print 'yes'
else:
    print 'no'

What you get will be no , actually x has been changed, but it is still False

if not x is None and if x is not None have the same result.

if not x is None => if not (x is None)
if x is not None => if x (is not) None